Answer
Use a SQL-statement-based custom field instead of a Reference-type one — call the appropriate API getter function for the value you need, parameterized using the current form's own bind variables (such as :order_no and :contract), following the general pattern SELECT <Some_API>.Get_Some_Value(:bind_var) FROM dual. For a Purchase Order header's Coordinator-style field specifically, this means calling the relevant Purchase Order API accessor function with the current record's own order number as the bind variable, rather than trying to configure a direct object reference between two otherwise-unrelated Logical Units.